Abstract:

A method is provided for performing plasma immersion ion implantation with
a highly uniform seasoning film on the interior of a reactor chamber
having a ceiling and a cylindrical side wall and a wafer support pedestal
facing the ceiling. The method includes providing a gas distribution ring
with plural gas injection orifices on a periphery of a wafer support
pedestal, the orifices facing radially outwardly from the wafer support
pedestal. Silicon-containing gas is introduced through the gas
distribution orifices of the ring to establish a radially outward flow
pattern of the silicon-containing gas. The reactor includes pairs of
conduit ports in the ceiling adjacent the side wall at opposing sides
thereof and respective external conduits generally spanning the diameter
of the chamber and coupled to respective pairs of the ports. The method
further includes injecting oxygen gas through the conduit ports into the
chamber to establish an axially downward flow pattern of oxygen gas in
the chamber. RF power is coupled into the interior of each of the
conduits to generate a toroidal plasma current of SixOy species
passing through the chamber to deposit a seasoning layer of a
SixOy material on surfaces within the chamber, while leaving
the pedestal without a wafer so as to expose a wafer support surface of
the pedestal.

Claims:

1. A method for performing plasma immersion ion implantation in a reactor
chamber having a ceiling and a cylindrical side wall and a wafer support
pedestal facing the ceiling, comprising:providing a gas distribution ring
with plural gas injection orifices on a periphery of a wafer support
pedestal, said orifices facing radially outwardly from said wafer support
pedestal;introducing a silicon-containing gas through the gas
distribution orifices of said ring to establish a radially outward flow
pattern of said silicon-containing gas;providing pairs of conduit ports
in said ceiling adjacent the side wall at opposing sides thereof and
providing respective external conduits generally spanning the diameter of
said chamber and coupled to respective pairs of said ports;injecting
oxygen gas through the conduit ports into said chamber to establish an
axially downward flow pattern of oxygen gas in the chamber;coupling RF
power into the interior of each of said conduits to generate a toroidal
plasma current of SixOy species passing through said chamber to
deposit a layer of a SixOy material on surfaces within said
chamber, while leaving said pedestal without a wafer so as to expose a
wafer support surface of the pedestal;placing a wafer on the
pedestal;introducing ion implantation precursor gases into the chamber
through a gas distribution plate that extends across the ceiling;
andcoupling RF power into the interior of each of said conduits to
generate a toroidal plasma of implant species current passing through
said chamber so as to implant said implant species into said wafer.

2. The method of claim 1 further comprising applying RF bias power to an
electrode to produce a plasma sheath bias voltage on the order of
kilovolts.

3. The method of claim 1 wherein said pedestal comprises an electrostatic
chuck providing a wafer support surface, said method further comprising
enhancing the electrical conductivity of the portion of the seasoning
layer deposited on said wafer support surface sufficiently to provide a
conductive path for discharging the wafer to electrostatically de-clamp
the wafer from the wafer support surface.

4. The method of claim 3 wherein the step of enhancing the electrical
conductivity comprises adjusting the flow rates of said
silicon-containing gas and said oxygen gas in said chamber so as to
deposit a silicon-rich form of SixOy on the wafer support
surface of said pedestal and an oxygen-rich form of SixOy on
chamber surfaces near said side wall.

5. The method of claim 4 wherein the step of adjusting comprises injecting
a silicon-containing gas through the gas distribution plate.

6. The method of claim 1 further comprising orienting said orifices of
said gas distribution ring in a direction toward the wafer support plane
of said pedestal.

8. A method for performing plasma immersion ion implantation in a reactor
chamber having a ceiling and a cylindrical side wall and a wafer support
pedestal facing the ceiling, comprising:introducing a silicon-containing
gas through radially facing gas distribution orifices at the side of a
gas distribution pedestal to establish a radially outward flow pattern of
said silicon-containing gas;injecting oxygen gas through conduit ports of
reentrant conduits of said chamber to establish an axially downward flow
pattern of oxygen gas in a peripheral region of the chamber;coupling RF
power into the interior of each of said conduits to generate a toroidal
plasma current of SixOy species passing through said chamber to
deposit a layer of a SixOy material on surfaces within said
chamber, while leaving said pedestal without a wafer so as to expose a
wafer support surface of the pedestal;placing a wafer on the
pedestal;introducing ion implantation precursor gases into the chamber
through a gas distribution plate that extends across the ceiling;
andcoupling RF power into the interior of each of said conduits to
generate a toroidal plasma of implant species current passing through
said chamber so as to implant said implant species into said wafer.

9. The method of claim 8 further comprising applying RF bias power to an
electrode to produce a plasma sheath bias voltage on the order of
kilovolts.

10. The method of claim 8 wherein said pedestal comprises an electrostatic
chuck providing a wafer support surface, said method further comprising
enhancing the electrical conductivity of the portion of the seasoning
layer deposited on said wafer support surface sufficiently to provide a
conductive path for discharging the wafer to electrostatically de-clamp
the wafer from the wafer support surface.

11. The method of claim 10 wherein the step of enhancing the electrical
conductivity comprises adjusting the flow rates of said
silicon-containing gas and said oxygen gas in said chamber so as to
deposit a silicon-rich form of SixOy on the wafer support
surface of said pedestal and an oxygen-rich form of SixOy on
chamber surfaces near said side wall.

12. The method of claim 11 wherein the step of adjusting comprises
injecting a silicon-containing gas through the gas distribution plate.

13. The method of claim 8 further comprising angling said radially
outwardly facing orifices in an upward direction toward the wafer support
plane of said pedestal whereby to establish an axial component in said
radially outward flow pattern.

Description:

BACKGROUND

[0001]Plasma immersion ion implantation of a semiconductor wafer is
typically used to form P--N junctions in the wafer surface. The plasma
immersion ion implantation (P3i) process is faster or more productive
than other implantation processes. In order to attain a requisite
implantation or junction depth, ion energy at the wafer surface must
relatively high, which can be accomplished by applying a sufficiently
high RF bias power to the wafer, or to an electrode within the wafer
support pedestal. The P3i reactor chamber is typically constructed of
aluminum components whose surfaces are anodized to provide some
protection and from plasma in the chamber. One problem is that the high
ion energy of the plasma during ion implantation produces ion bombardment
of the metallic chamber components, removing metal particles that
vaporize into the plasma to spread throughout the chamber and deposit on
the wafer. The high ion energy is attained by coupling RF bias power to
the wafer at a sufficient level to create a plasma bias voltage on the
order of tens or hundreds of kilovolts. Such metal contamination of the
wafer can produce defects in the devices formed on the wafer surface.

SUMMARY OF THE INVENTION

[0002]A method is provided for performing plasma immersion ion
implantation with a highly uniform seasoning film on the interior of a
reactor chamber having a ceiling and a cylindrical side wall and a wafer
support pedestal facing the ceiling. The method includes providing a gas
distribution ring with plural gas injection orifices on a periphery of a
wafer support pedestal, the orifices facing radially outwardly from the
wafer support pedestal. Silicon-containing gas is introduced through the
gas distribution orifices of the ring to establish a radially outward
flow pattern of the silicon-containing gas. The reactor includes pairs of
conduit ports in the ceiling adjacent the side wall at opposing sides
thereof and respective external conduits generally spanning the diameter
of the chamber and coupled to respective pairs of the ports. The method
further includes injecting oxygen gas through the conduit ports into the
chamber to establish an axially downward flow pattern of oxygen gas in
the chamber. RF power is coupled into the interior of each of the
conduits to generate a toroidal plasma current of SixOy species
passing through the chamber to deposit a seasoning layer of a
SixOy material on surfaces within the chamber, while leaving
the pedestal without a wafer so as to expose a wafer support surface of
the pedestal. Upon completion of seasoning layer deposition, a wafer is
placed on the pedestal. Ion implantation precursor gases are introduced
into the chamber through a gas distribution plate that extends across the
ceiling. Plasma immersion ion implantation is performed by coupling RF
power into the interior of each of the conduits to generate a toroidal
plasma of implant species current passing through the chamber so as to
implant the implant species into the wafer. The method can further
include applying RF bias power to an electrode underlying the wafer to
produce a plasma sheath bias voltage on the order of kilovolts.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003]So that the manner in which the above recited embodiments of the
invention are attained and can be understood in detail, a more particular
description of the invention, briefly summarized above, may be had by
reference to the embodiments thereof which are illustrated in the
appended drawings. It is to be noted, however, that the appended drawings
illustrate only typical embodiments of this invention and are therefore
not to be considered limiting of its scope, for the invention may admit
to other equally effective embodiments.

[0004]FIG. 1 is a cut-away side view of a plasma reactor in accordance
with one aspect.

[0007]FIG. 4 is a graph depicting the variations in stochiometry of a
seasoning layer controlled in accordance with the gas flow patterns of
FIG. 3.

[0008]FIG. 5 is a diagram depicting a process performed by the reactor of
FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0009]In order to minimize or prevent metal contamination from occurring
during the P3i process, the chamber interior surfaces can be coated with
a non-metallic "seasoning" film prior to the plasma immersion ion
implantation and prior to introduction of the wafer into the chamber. The
ideal thickness of the seasoning film, at which metal contamination is
reduced below specified limits, is readily determined using empirical
methods. Typically, the minimum thickness is on the order of 1000 Å,
although better result are obtained at more ideal thicknesses, such as
2000 Å. The seasoning film must be removed following the P3i process
and thereafter replaced because some of the film is removed--or its
thickness reduced--in a non-uniform manner during the P3i process, so
that it may not offer adequate protection from metal contamination during
a subsequent P3i step. This is particularly true of cases in which the
ion implantation plasma is formed of a gas including a fluoride compound
of the ion implantation species. The seasoning film removal step can be
carried out by filling the chamber with seasoning-removal gas species
obtained from an external ("downstream") plasma source. Such gases may be
corrosive species such as fluorine-containing compounds, for example.

[0010]The seasoning film is deposited using a high density plasma enhanced
chemical vapor deposition (HDPCVD) process by introducing a
silicon-containing gas (e.g., silane) and oxygen gas into the reactor
chamber and igniting a plasma. Radicals, neutrals and/or ions of
silicon-oxygen compounds are formed in the plasma, which deposit on the
interior chamber surfaces to form a thin film or coating of SiO2
and/or SixOy, for example. The problem is that the thickness of
the seasoning film is highly non-uniform because of non-uniformities in
gas flow, non-uniform RF power or field distribution throughout the
chamber and obstructions offered by some mechanical features in the
chamber interior. For chamber surfaces in areas of lower gas flow, where
the seasoning film deposition rate is slowest, the minimum required
seasoning film thickness (e.g., 1000 Å) is not reached until an
excessive seasoning film thickness (e.g., 12,000 Å) is reached in
other areas of high gas flow and the highest deposition rates. The result
is that the seasoning deposition step takes longer than it
should--depressing productivity. Moreover, the post-implant seasoning
removal process exposes the chamber interior surfaces where the seasoning
film was thinnest (1000 Å) well before removal of the thicker (12,000
Å) portions of the seasoning film. The chamber surfaces first exposed
during the post-implant seasoning removal step are therefore attacked by
the cleaning gases during the remainder of the cleaning step, shortening
the life of those components and increasing the operating cost of the
reactor.

[0011]A related problem arises from the non-uniform distribution of the
removal rate of the seasoning film during the P3i process and during the
post-implant seasoning removal (clean) process. The removal rate during
the P3i step is not uniform, because the P3i plasma is concentrated
primarily in the wafer-ceiling gap or process zone. In cases, for
example, where the seasoning precursor gas is introduced from the chamber
bottom, bottom-facing surfaces of some chamber components, such as radial
struts supporting the wafer pedestal, receive the thickest coating.
Unfortunately, these surfaces face away from the main plasma (i.e., away
from the wafer-ceiling gap) and therefore receive little ion bombardment,
and are therefore the least reduced in thickness during the P3i step.
Having started out with the greatest seasoning thickness and then having
been attacked the least during the P3i step, such surfaces bear a
disproportionately thick seasoning film and are therefore difficult to
clean without undue exposure of other chamber surfaces to corrosive
cleaning gases.

[0012]FIG. 1 illustrates a toroidal source plasma reactor for plasma
immersion ion implantation having gas distribution features that solve
the foregoing problems. These gas distribution features are used during
the pre-implant seasoning deposition step to form a seasoning film
throughout the chamber interior that is highly uniform. Some of the
features may be used to control the stochiometry of the seasoning film to
achieve desired characteristics, as will be discussed herein.

[0013]The reactor of FIG. 1 includes a cylindrical chamber 100 defined by
a cylindrical side wall 102, a ceiling 104 and a floor 106. A wafer
support pedestal 108 includes an electrostatic chuck 110 having a wafer
support surface for holding a semiconductor wafer 112. The ceiling 104
has two pairs of openings 114 to which respective mutually orthogonal
external reentrant conduits 116, 118 are coupled. Each conduit 116, 118
completes a closed reentrant path for an oscillating plasma current
passing through the process region defined by the gap between the
pedestal 108 and the ceiling 104. The ceiling 104 is a gas distribution
plate having an array of gas injection orifices 120 facing the chamber
interior and an interior gas distribution manifold 122. Optionally, the
manifold 122 may be divided into radially inner and outer portions 122a,
122b, to establish independent inner and outer gas injection zones (or
groups) 124a, 124b of the orifices 122. In this case, a pair of
separately controlled gas supplies 126a, 126b are coupled to the inner
and outer manifolds 122a, 122b. A pair of plasma RF source power
generators 128, 130 are coupled to apply RF power to the interiors of
respective ones of the conduits 116, 118 via respective impedance matches
132, 134 and power applicators 136, 138. Each power applicator 136, 138
may be of the same structure which consists of a magnetically permeable
core or ring 140 wrapped around the respective conduit 116 or 118, and a
conductive coil 142 wrapped around the ring 140. The electrostatic chuck
(ESC) 110 consists of a conductive electrode 110a and an insulator layer
110b in which the electrode 110a is contained. An RF bias power generator
142 is coupled to the ESC electrode 110a through an impedance match 144.
A D.C. chuck voltage supply 146 is coupled to the ESC electrode 110a. The
pedestal 108 is supported on three radial struts 150-1, 150-2, 150-3,
best shown in FIG. 2, that extend inwardly from the side wall 102 and
underneath the pedestal 108.

[0014]During plasma immersion ion implantation, an implant species
precursor gas, such as a boron fluoride or a boron hydride in the case of
a boron implantation step, is injected through the ceiling gas
distribution plate 104 while plasma source power is applied by the
generators 128, 130 to produce an oscillating closed plasma current in
the reentrant path through the process region overlying the wafer. For
this purpose, an ion implantation process gas supply 180 is coupled to
the inner and outer gas manifolds 122a, 122b of the gas distribution
plate 104. Optionally, the RF generator 142 applies bias power to the ESC
110 to control ion energy and (hence) implant depth. The gas distribution
plate 104 is optimized for uniform gas distribution across the surface of
the wafer or wafer support surface of the ESC 110, but is not structured
for uniform gas distribution throughout the chamber interior. Therefore,
the gas distribution plate 104 is not, by itself, suitable for use in
depositing the seasoning film throughout the chamber.

[0015]The gas distribution features that provide for a uniform seasoning
film in the chamber include a center array of gas injection orifices 202
along the side wall of the pedestal 108 for injecting the
silicon-containing gas. The center array of orifices 202 is formed in a
hollow gas distribution ring 200 supported on the side wall of the
pedestal 108 and extending around the periphery of the pedestal. In the
illustrated embodiment, the pedestal 108 supports an electrostatic chuck
(ESC) 110. The ESC includes a conductive base 204 underlying the
insulating layer 110. The base 204 may include internal features for
utilities such as coolant passages and backside gas flow passages (not
shown). In the illustrated embodiment, the gas distribution ring 200 is
attached to the outer periphery of the base 204. The gas injection
orifices 202 on the ring 200 may be oriented at an angle A relative to
the horizontal wafer plane so as to inject the silicon-containing gas at
an upward direction. This promotes better deposition on upward facing
surfaces of interior chamber features, such as the wafer support surface
of the ESC 110 and the top surfaces of the radial struts 150, for
example. A silicon-containing (e.g., silane) gas supply 206 is connected
to the hollow interior of the gas distribution ring 200.

[0016]The oxygen gas is injected during the seasoning film deposition step
through the four conduit ports 114 in the ceiling 104. For this purpose,
conduit injection orifices 210 inject gas into the conduits 116, 118 near
each of the conduit ports 114. An oxygen gas supply 212 is coupled to
each of the conduit injection orifices. Oxygen injection through the
conduit ports 114 promotes a more oxygen-rich gas mixture near the sides
of the chamber and, therefore, a more silicon-rich gas mixture over the
center of the chamber, i.e., over the wafer support surface of the ESC
110. This is because the conduit ports 114 are all located near the
periphery of the ceiling 104.

[0017]During the pre-implant seasoning film deposition step, the wafer 112
is absent and no implant process gases are supplied to the gas
distribution plate 104. Therefore, the gas distribution plate 104 is
available for use during the seasoning film deposition step. Optionally,
the gas distribution plate 104 may be exploited during the seasoning film
deposition to control the thickness distribution and the stochiometry
distribution of the seasoning layer. For example, a further increase in
the silicon content of the gases over the center of the chamber (over the
pedestal 108) can be realized by injecting the silicon-containing gas
(silane) through the center (inner) gas distribution zone 124 of the gas
distribution plate 104. For this purpose, the inner zone gas supply 126a
stores silane, for example. To promote a thicker film on upward facing
surface without detracting from the predominance of silicon-containing
gas at the center, oxygen gas could be injected through the outer gas
injection zone 124b of the gas distribution plate 104. In this case, the
outer zone gas supply 126b stores oxygen gas. The proportion of silicon
to oxygen in the center and periphery of the chamber is controlled or
affected by the different gas flow rates to the inner and outer zones
122a, 122b as well as the gas flow rates to the conduit injection
orifices 210 and to the gas distribution ring 200. FIG. 3 summarizes the
foregoing gas flows by species from each of the gas injection elements of
the reactor of FIG. 1.

[0018]The position (e.g., axial height) of the gas distribution ring 200
and the angle A of the orifices 202 of the gas distribution ring 200 can
be adjusted to achieve a desired uniformity of the coating of
SixOy. Oxygen and SiH4 can be supplied at a controlled rate to
the distribution ring 200 for injection through the orifices 202 in order
to control the stochiometry of the coating or seasoning deposition.

[0019]We have found that the foregoing features solve the problem
non-uniform seasoning deposition. Whereas prior to the invention the
seasoning thickness varied from a minimum of about 1000 Å on some
upward facing surfaces to a maximum of about 12,000 Å on some
downward facing surfaces, the invention produces a much smaller variation
in seasoning thickness, permitting us to establish a much greater minimum
thickness (of about 2000 Å) without exceeding a maximum of about
3000-4000 Å. This greater minimum thickness is achieved in a much
shorter deposition time, while the removal step is performed very
quickly, thus increasing throughput.

[0020]FIG. 4 is a graph depicting a desired radial distribution of Si--O
stochiometry of the seasoning layer that can be controlled with the
foregoing gas distribution features. Specifically, by providing more
silane (silicon-containing) gas over the wafer pedestal 108 and more
oxygen gas at the periphery, the stochiometry of the SixOy
seasoning film (i.e., the ratio x:y) is distributed so as to have a
silicon-rich proportion at the center and a silicon-lean proportion at
the periphery. The advantage is that a material with a higher
conductivity is provided where it is needed, i.e., on the ESC 110. The
higher conductivity of the silicon-rich mixture covering the ESC 110
enables the electric charge on the wafer to be removed more rapidly
during de-chucking of the wafer, leading the better throughput or
productivity. By providing a more conductive path for charge on the wafer
to dissipate, the electrostatic clamping force holding the wafer to the
ESC 110 is more rapidly removed when the D.C. chucking voltage source is
switched off to dechuck the wafer. In FIG. 4, the proportion (x) of
silicon is maximum over the pedestal 108 while the proportion (y) of
oxygen is minimum in the same area. The relationship is reversed at the
periphery, as indicated in the graph of FIG. 4. A nominal value for x is
1 while a nominal value for y is 2. The stochiometric variation
illustrated in the graph of FIG. 4 between the radially inner and outer
regions of the deposited seasoning film is increased by increasing the
flow of the silicon-containing gas to the center (e.g., through the gas
distribution plate 104 or through the gas distribution ring 200) relative
to the oxygen gas flow to the periphery (e.g., through the conduit ports
114).

[0021]FIG. 5 depicts a process involving pre-implant seasoning deposition,
plasma immersion ion implantation and post-implant cleaning or seasoning
removal. The entire cycle begins with no wafer on the pedestal 108 (block
250 of FIG. 5). For pre-implant seasoning deposition, a
silicon-containing gas (e.g., silane) is injected through gas injection
orifices 202 around the side of the pedestal 108 (block 252) and oxygen
gas is injected through the ceiling ports 114 of the external reentrant
conduits 116, 118 (block 254). An optional step (block 256) is to inject
either silane or oxygen through ceiling gas distribution plate 104. In
the case of silane, the flow rate is adjusted to achieve about a 2-10%
enhancement of the silicon content of the seasoning layer deposited on
the ESC 110 over the nominal 1:2 silicon-to-oxygen ratio of silicon
dioxide. A plasma is generated (by applying RF power either to the
applicators 136, 138 or to the ESC electrode 110a, to deposit a
SixOy seasoning film on chamber interior surfaces (block 258).
An optional step (block 260) is to adjust gas flow (e.g., either silane
or oxygen) through ceiling gas distribution plate 104 to achieve desired
enrichment of silicon proportion (x:y) of seasoning film on the wafer
support surface of the electrostatic chuck, in accordance with FIG. 4.
Then, the seasoning deposition process is stopped after desired seasoning
film thickness (e.g., 2000 Å) has been reached (block 262). A wafer
is placed onto the pedestal 108 (block 264). Plasma immersion ion
implantation is performed (block 266) by introducing an implant
species-containing process gas and applying RF source power to the
conduit RF power applicators 136, 138. During this step, ion energy
(implant depth) may be controlled by applying RF bias power to the ESC
electrode 110a from the generator 142. Upon completion of the implant
step, wafer is removed (block 268). The seasoning film is removed (block
270) by introducing a seasoning layer etch gas from a downstream plasma
source, for example.

[0022]While the foregoing is directed to embodiments of the invention,
other and further embodiments of the invention may be devised without
departing from the basic scope thereof, and the scope thereof is
determined by the claims that follow.